The present invention relates to a composite prepreg or woven material, and more particularly, to a composite material with improved resistance to core crush and porosity.
Structural composite parts of aircraft designed with honeycomb core for stiffening and joggled flanges (such as ribs, spars, elevators, rudders, flaps, etc.) frequently experience producibility problems associated with these two design elements. Honeycomb core in composite parts can experience xe2x80x9ccore crushxe2x80x9d which is a non-repairable defect that occurs when honeycomb core sections collapse. Core crush is thought to be related to the properties of the prepreg and woven composite materials. Composite prepreg materials contain a fiber reinforcement form (usually tape or fabric) that has been preimpregnated with a liquid resin and thermally advanced to a viscous stage. Composite woven materials contain interlaced yarns or fibers, usually in a planar structure, that establish a weave pattern from the yarns which is used as the fibrous constituent in an advanced composite lamina.
Parts with joggled flanges are also sensitive to porosity in the joggle region due to the inability of the prepreg to stay xe2x80x9cseatedxe2x80x9d against the radius, and the joggle of the tool during lay-up and cure. Porosity is a defect involving unfilled space inside a material that frequently limits the material strength.
These core crush and porosity defects are producibility problems that are currently experienced worldwide. Core crush and porosity are the two predominant types of defects leading to part rejections in prepreg and woven composite materials since these conditions can be rarely be repaired.
Extensive research and development has been performed over the years by composite part fabricators in an effort to solve the core crush producibility problem. Core details and adjacent prepreg plies are stabilized in current production parts by various different methods (ply tie-downs, precured adhesive over the core, etc.) to reduce this core crush problem. Specific stabilization methods are documented in The Boeing Company""s composite BAC Process Specifications which are incorporated herein by reference. However, these stabilization methods are unsatisfactory in that they are time consuming and add significant expense to the current production of sandwiched structure parts.
Likewise, extensive research and development has been performed in an attempt to address the porosity producibility problem in joggled parts. Particularly those parts utilizing the Boeing BMS 8-256 prepreg material (as described in the Boeing Materials Specification incorporated herein by reference). The extremely low flow properties of this prepreg""s resin have particularly exacerbated the problem of porosity in parts designed with joggles. The BMS 8-256 prepreg material is currently one of the most widely used prepreg materials for composite secondary and primary structures for aircraft. Both material and process improvements have been evaluated in an effort to eliminate porosity. These have included the use of elastomeric pressure pads against the joggle during cure, decreasing part staging time prior to the cure, increasing the tack and drape of the prepreg, etc. These measures have yet to totally and reliably eliminate porosity in the joggles of parts fabricated with a prepreg material having low flow resin properties.
There is a continuing need in the art for a structural composite material designed with a honeycomb core that is resistant to core crush and porosity defects, particularly for a material having high resin viscosity and/or low flow properties.
The present invention is directed towards a composite material that includes warp yarns and fill yarns. The warp and fill yarns are composed of at least two different kinds of yarn that are selected from the group consisting of standard twist fiber (ST), untwisted fiber (UT), and never twisted fiber (NT). Many different combinations of ST fiber, UT fiber, and NT fiber are possible for utilization in the warp and the fill, as described with greater specificity below.
In a preferred embodiment of the present invention, the warp yarns comprise one of the group consisting of standard twist fiber, untwisted fiber, and never twisted fiber; and the fill yarns comprise a different one of the group consisting of standard twist fiber, untwisted fiber, and never twisted fiber. Thus, in one version of this preferred embodiment, the warp yarns comprise one of the group consisting of standard twist fiber and never twisted fiber, and the fill yarns comprise the other of the group consisting of standard twist fiber and never twisted fiber. In another version of this preferred embodiment, the warp yarns comprise one of the group consisting of untwisted fiber and never twisted fiber, and the fill yarns comprise the other of the group consisting of untwisted fiber and never twisted fiber. In yet another version of this preferred embodiment, the warp yarns comprise one of the group consisting of standard twist fiber and untwisted fiber, and the fill yarns comprise the other of the group consisting of standard twist fiber and untwisted fiber.
In another preferred embodiment of the present invention, a first percentage of the warp yarns comprise one of the group consisting of standard twist fiber, untwisted fiber, and never twisted fiber; and a second percentage of the warp yarns comprise a different one of the group consisting of standard twist fiber, untwisted fiber, and never twisted fiber. Thus, in one version of this preferred embodiment, a first percentage of the warp yarns comprise one of the group consisting of standard twist fiber and never twisted fiber, and a second percentage of the warp yarns comprise the other of the group consisting of standard twist fiber and never twisted fiber. In another version of this preferred embodiment, a first percentage of the warp yarns comprise one of the group consisting of standard twist fiber and untwisted fiber, and a second percentage of the warp yarns comprise the other of the group consisting of standard twist fiber and untwisted fiber. In still another version of this preferred embodiment, a first percentage of the warp yarns comprise one of the group consisting of untwisted fiber and never twisted fiber, and a second percentage of the warp yarns comprise the other of the group consisting of untwisted fiber and never twisted fiber.
In another aspect of a preferred embodiment of the present invention, a first percentage of the fill yarns comprise one of the group consisting of standard twist fiber, untwisted fiber, and never twisted fiber; and a second percentage of the fill yarns comprise a different one of the group consisting of standard twist fiber, untwisted fiber, and never twisted fiber. Thus, in one version of this preferred embodiment, a first percentage of the fill yarns comprise one of the group consisting of standard twist fiber and never twisted fiber, and a second percentage of the fill yarns comprise the other of the group consisting of standard twist fiber and never twisted fiber. In another version of this preferred embodiment, a first percentage of the fill yarns comprise one of the group consisting of untwisted fiber and never twisted fiber, and a second percentage of the fill yarns comprise the other of the group consisting of untwisted fiber. and never twisted fiber. In still another version of this preferred embodiment, a first percentage of the fill yarns comprise one of the group consisting of standard twist fiber and untwisted fiber, and a second percentage of the fill yarns comprise the other of the group consisting of standard twist fiber and untwisted fiber.
In still another preferred embodiment of the present invention, the warp yarns comprise two of the group consisting of standard twist fiber, untwisted fiber, and never twisted fiber; and the fill yarns comprise two of the group consisting of standard twist fiber, untwisted fiber, and never twisted fiber. In one version of this preferred embodiment, the warp yarns comprise two of the group consisting of standard twist fiber, untwisted fiber, and never twisted fiber; and the fill yarns comprise the same two of the group consisting of standard twist fiber, untwisted fiber, and never twisted fiber.
In yet another preferred embodiment of the present invention, the warp yarns comprise one of the group consisting of standard twist fiber, untwisted fiber, and never twisted fiber; and the fill yarns comprise the two of the group consisting of standard twist fiber, untwisted fiber, and never twisted fiber. In one version of this preferred embodiment, the warp yarns comprise one of the group consisting of standard twist fiber, untwisted fiber, and never twisted fiber; and the fill yarns comprise the other two of the group consisting of standard twist fiber, untwisted fiber, and never twisted fiber.
One preferred embodiment of the present invention includes warp yarns that comprise two of the group consisting of standard twist fiber, untwisted fiber, and never twisted fiber; and fill yarns that comprise the one of the group consisting of standard twist fiber, untwisted fiber, and never twisted fiber. A version of this preferred embodiment includes warp yarns that comprise two of the group consisting of standard twist fiber, untwisted fiber, and never twisted fiber, and fill yarns that comprise the other one of the group consisting of standard twist fiber, untwisted fiber, and never twisted fiber.
In an alternate preferred embodiment of the present invention, a first percentage of the warp yarns comprise one of the group consisting of standard twist fiber, untwisted fiber, and never twisted fiber; a second percentage of the warp yarns comprise a different one of the group consisting of standard twist fiber, untwisted fiber, and never twisted fiber; and a third percentage of the warp yarns comprise a remaining one of the group consisting of standard twist fiber, untwisted fiber, and never twisted fiber. In another aspect of this alternate preferred embodiment, a first percentage of the fill yarns comprise one of the group consisting of standard twist fiber, untwisted fiber, and never twisted fiber; a second percentage of the fill yarns comprise a different one of the group consisting of standard twist fiber, untwisted fiber, and never twisted fiber; and a third percentage of the fill yarns comprise a remaining one of the group consisting of standard twist fiber, untwisted fiber, and never twisted fiber.
In another alternate preferred embodiment of the present invention, the warp yarns comprise two of the group consisting of standard twist fiber, untwisted fiber, and never twisted fiber; and the fill yarns comprise all three of the group consisting of standard twist fiber, untwisted fiber, and never twisted fiber. Another preferred embodiment includes warp yarns that comprise all three of the group consisting of standard twist fiber, untwisted fiber, and never twisted fiber; and fill yarns that comprise two of the group consisting of standard twist fiber, untwisted fiber, and never twisted fiber.
In yet another alternate preferred embodiment of the present invention, the warp yarns comprise all three of the group consisting of standard twist fiber, untwisted fiber, and never twisted fiber; and the fill yarns comprise all three of the group consisting. of standard twist fiber, untwisted fiber, and never twisted fiber. Preferably, the composite material of the present invention is prepreg composite material, and the fiber of the present invention is carbon fiber. Additionally, the standard twist fiber has a substantially circular cross-section, the never twisted fiber has a substantially elliptical cross-section, and the untwisted fiber has a modified elliptical cross-section.
Another exemplary embodiment of the present invention contains multi-directional fibers having at least first and second directional configurations of interlaced material, which in turn include at least two different kinds of yarn selected from the group consisting of standard twist fiber, untwisted fiber, and never twisted fiber. An additional exemplary embodiment of the present invention contains a material of warp fiber tows and fill fiber tows. The warp and fill tows include at least two different kinds of fiber, the first of the at least two different kinds of fiber having an approximately circular cross-section, a lower degree of spreadability, and a higher degree of frictional resistance, the second of the at least two different kinds of fiber having an approximately elliptical cross-section, a higher degree of spreadability, and a lower degree of frictional resistance. The combination of at least two different kinds of yarn selected from the group facilitates reducing the frequency of porosity and core crush defects.